1. Technical Field
The present invention relates to an apparatus and method for heating a water or petroleum based fluid for injection into an oil or gas well or into a pipeline system.
2. Description of the Related Art
It is common in the oil and gas industry to treat oil and gas wells and pipelines with heated fluids such as water and oil. For example, one such application commonly known as a hydraulic fracturing job or “frac” job, involves injecting large quantities of a heated aqueous solution into a subterranean formation to hydraulically fracture it. Such frac jobs are typically used to initiate production in low-permeability reservoirs and/or re-stimulate production in older producing wells. Water is typically heated to a specific temperature range to prevent expansion or contraction of the downhole well casing. The heated water is typically combined with a mixture of chemical additives (e.g., friction reducer polymers which reduce the viscosity of the water and improve its flowability so that it's easier to pump down the well), proppants (e.g., a special grade of light sand), and a cross-linked guar gel that helps to carry the sand down into the well. This fraccing fluid is then injected into a well hole at a high flow rate and pressure to break up the formation, increasing the permeability of the rock and helping the gas or oil flow toward the surface. As the fraccing solution cracks the rock formation, it deposits the sand. As the fractures try to close, the sand keeps them propped open. Frac jobs are typically performed once when a well is newly drilled, and again after a couple of years when the production flow rate begins to decline
Another application, commonly referred to as a “hot oil treatment”, involves treating tubulars of an oil and gas well or pipeline by flushing them with a heated solution to remove build up of paraffin along the tubulars that precipitate from the oil stream that is normally pumped therethrough.
Frac jobs and hot oil treatments are typically performed at the remote well sites and usually require less than a week to complete. Consequently, the construction of a permanent heating facility at the well site is not cost effective. Instead, portable heat exchangers, which are capable of transport to remote well sites via improved and unimproved roads, are commonly used.
In the past, such portable heat exchangers have typically employed gas-fired heat sources using a liquefied petroleum gas (LPG) such as propane to heat treatment fluids at remote well sites. Such gas-fired heater units typically include a tubular coil heat exchanger configured above one or more open flame gas burners in an open-ended firebox housing. The tubular coil heat exchanger typically comprises a fluid inlet in communication with a plurality of interconnected tubes, which in turn communicate with a fluid outlet. The plurality of tubes are typically arranged in a stacked configuration of planar rows, wherein each tube in a row is aligned in parallel with the other tubes. The outlet of each tube is connected in series to the inlet of an adjacent tube in the row by means of a curved tube or return bend. Similarly, each planar row is connected to the adjacent rows above and below by connecting the outlet of the outermost tube in one row with the inlet of the outermost tube in another row by means of a curved tube or return bend.
The one or more gas burners are typically positioned below the tubular coil heat exchanger so as to project a vertical flame up and through the heat exchanger. The gas burners are supplied with gas fuel from a nearby gas storage tank (e.g., a propane tank). Ambient air is also supplied to the burners via the opened-ended bottom of the firebox housing. The hot flue gasses generated from the burning of the LPG rise up and through the tubular coil heat exchanger within the firebox housing and exhaust via a vent at the top of the firebox housing.
While gas-fired heat sources are adequate for performing many oil field servicing tasks, they exhibit a number of inherent drawbacks. These inherent limitations significantly impact their effectiveness in performing certain heating operations at remote oil field work sites. For example, frac jobs typically require the production of massive volumes of heated water. While gas-fired heat sources are certainly capable of heating fluids such as water, they are poorly suited to heating in a timely manner large volumes of continuously flowing water in many commonly occurring climactic and atmospheric conditions. Moreover, the logistics involved in conducting such heating operations at remote work sites negatively impacts the cost efficiencies of such a system.
For example, LPG (e.g., propane gas) has a relatively low energy content and density when compared to other fuel options. For example, diesel fuel when properly combusted typically releases about 138,700 British thermal units (BTU) per US gallon, while propane typically releases only about 91,600 BTU per liquid gallon, or over 33% less. Thus, gas-fired heating units often lack sufficient heating capacity to produce sufficient quantities of heated water rapidly enough for the required operation to be completed. Consequently, in order to provide sufficient quantities of heated water on a timely basis for a typical frac job, the treatment water must often be preheated and stockpiled in numerous frac water holding tanks. These holding tanks range in size up to 500 bbl (i.e., approximately 21,000 gallons). It is not unusual for a typical frac job to require 10 or even 20 frac water holding tanks at the remote work site. The preheated water is typically overheated so as to allow for cooling while waiting to be injected into the well. Oftentimes, the preheated treatment water must be reheated just prior to injection into the well head. Needless to say, the logistics involved with providing additional holding tanks at the remote work site and the additional costs incurred in overheating or reheating the supply water negatively impacts the efficiency of the overall operation.
While the technique of overheating and stockpiling supply water can ameliorate some the shortcomings in the heating capacity of gas-fired heat sources, in certain circumstances (e.g., severely cold weather or high altitude) it is inadequate. This is due to a number of reasons. First, the temperature change requirement for the system is simply greater in colder weather. That is, in colder weather the intake water supplied to the gas-fired heating unit is colder while the required injection temperature remains essentially the same. Thus, it takes longer for the gas-fired heating unit to preheat the supply water. The problem is further compounded by the fact that the stockpiled preheated water cools more rapidly in colder weather. Moreover, at higher altitudes there is less oxygen in the ambient atmosphere for combustion in the gas burner. Thus, at higher altitudes the heating capacity of gas-fired heat sources is further reduced.
In addition, propane gas requires large and heavy high-pressure fuel tanks for its transport to remote sites. The size of such high-pressure fuel tanks is, of course, limited by the size of existing roads. Thus, a typical frac job may require the transport of multiple large high-pressure fuel tanks to a remote site to ensure an adequate supply of fuel to complete the operation.
Furthermore, there are several safety concerns which must be taken into consideration when using gas-fired heat sources. As mentioned previously, current gas-fired heat exchangers typically use an open flame burner, i.e., a burner which is open to the ambient atmosphere. The fire boxes of such heat exchanger are typically elevated above the ground and opened on the bottom. The gas-fired burners are typically positioned near the open bottom of the firebox and directly below the heat exchange tubing. The gas-fired burners draw ambient air as necessary to assist in the combustion of the propane gas. While simple and efficient in providing air for combustion, open flame burners present a number of safety concerns. An open flame at the well site poses a substantial risk of explosion and uncontrolled fire, which can destroy the investment in the rig and injure or even cost the lives of the well operators. Moreover, open flame burners are particularly susceptible to erratic burning or complete blow-out in gusty wind conditions. Current U.S. government safety regulations provide that the open flame heating of the treatment fluids cannot take place within the immediate vicinity of the well.
While safety concerns are of overriding importance, compliance with the no open-flame regulations requires additional time and expense to conduct heated fluid well treatments. Thus, there has been a long felt need for a safer and more efficient apparatus and method of heating a treatment fluid for injecting into the tubulars of oil and gas wells and pipelines without using an open flame heat source in the vicinity of the treatment location.